Just to give you all a little more information- I did in fact ask straight out if when they started dating she had wanted/expected it to be monogamous. She did not answer directly, but instead said she knew from the start that this is just "who he is". Which to me sounded like she would have liked it to be monogamous, but was willing to try it another way to be with him. Could just be my interpretation, though.

We have talked all together about this a while back, but it was before I knew her very well. She confessed to having some issues with the situation, but she downplayed it as her own insecurities, and that this was part of being polyamorous- acknowledging that sometimes it's difficult, and that's okay.

Perhaps someone could give me insight into how this works when the primary couple isn't having a satisfying sex life? Is that common?

To answer the question about how I feel- I am happy in the relationship when I just think about it terms of me and him. But when I start thinking about his gf, I feel guilty, and sad that she is not experiencing the advantages of being in a poly relationship (at least, from my viewpoint). I am hyper-aware of my tendency to worry about other people to the point of putting their happiness before my own. But I care about him, and now her to some extent as well, and I don't want to do anything to hurt anyone.

I think you are all right about talking to them, or her alone. But I'm not sure I'm brave enough for that quite yet.